Contrast enhancement layer composition with naphthoquinone diazide, indicator dye and polymeric binder

ABSTRACT

This invention relates to a composition and method for enhancing the contrast of images of an underlying photolithographic layer. A photographic element, including a photoresist layer, is applied to a substrate. The photoresist layer contains a contrast-enhancing layer including at least one light-sensitive compound capable of producing acidic photoproducts upon exposure to actinic radiation, at least one indicator dye that changes color on exposure to acidic conditions and at least one polymeric binder soluble in water or weakly alkaline aqueous solutions. The photographic element is exposed to active radiation sufficient to cause photobleaching of the contrast enhancement layer and exposure of the photoresist layer. The photographic element is exposed to actinic radiation sufficient to cause photobleaching of the contrast enhancement layer and exposure of the photoresist layer. The photographic element is developed in an aqueous photoresist developer, whereby the contrast enhancement layer is stripped from the photoresist layer by the developer.

FIELD OF THE INVENTION

The present invention is directed to a composition and method forenhancing the contrast of images of objects such as masks forphotolithography. The invention is particularly useful in themanufacture of semiconductor devices for integrated circuits andcomponents thereof.

BACKGROUND IF THE INVENTION

Photolithography in the production of integrated circuits ispredominantly carried out by optical means. Photolithography is in manyways the key to microelectronic technology. It is involved repeatedly inthe processing of any device, at least once for each layer of thefinished structure. An important requirement of the lithographic processis that each pattern be positioned accurately with respect to the layersunder it. One technique is to hold the mask (which serves as a template)just off the surface and to visually align the mask with the existingpatterns in the wafer. After perfect alignment is achieved, the mask ispressed into contact with the wafer. The mask is then flooded withultraviolet radiation to expose the photoresist. The space between thewafer and the mask is often evacuated to achieve intimate contact.

Problems in the masking process arise from the need to print very smallfeatures with no defects in the pattern. If the mask were to bepositioned very far from the surface, diffraction of the ultravioletradiation passing through the mask would cause the smaller features toblur together. On the other hand, small particles on the wafer or themask are abraded into the mask when it is pressed against the wafer.Hence the masks can be used for only a few exposures before the abrasioncauses defects to accumulate to an intolerable level.

A more recent trend has been toward using the technique known asprojection alignment, in which the image of the mask is projected ontothe wafer through an optical system. In this case, mask life isvirtually unlimited. Projection alignment is now the predominant methodused in photolithographic production of semiconductors. However, theimage resolution of projection alignment lithographic systems isapproaching the physical limits imposed by practical constraints onnumerical aperture and wavelength. Although further improvements inlithographic technology are anticipated, dramatic improvements ininherent lens resolution are not. In order to continue the reduction ofminimum feature size achievable by optical techniques, it is necessaryto alter some other aspect of the lithographic process for furtherimprovements. The photoresist exposure and development process is onearea in which further improvements are possible.

A photoresist is a radiation-sensitive coating that is applied to asubstrate, exposed to an image, and developed by a process whichselectively removes (or leaves) the resist material that was exposed.For example, a negative photoresist may cross-link and polymerize uponexposure to ultraviolet or other types of radiation. Thus, exposure ofthe negative photoresist through a mask, followed by a development step(which consists of washing away the non-cross-linked material usingselective solvents) results in the removal of the photoresist whereverthe mask was opaque.

High contrast is necessary to produce on photoresists the image patternsused in the integrated circuit art. The minimum required contrast ofillumination is referred to as the "contrast threshold" of the resist.Depending on substrate properties, the required pattern thickness andresist edge profiles, a conventional positive photoresist has a contrastthreshold between about 85% and 90%. Currently, most production is doneat 90% contrast, or more. If the contrast threshold of the resist isreduced, the resolution obtainable with a given optical system isimproved due to the fact that image contrast is a decreasing function ofthe spatial frequencies present in the image.

Projection lithography generally uses an aerial image of a mask toexpose the photoresist. But as the contrast is reduced, discriminationof a darker area from a lighter area becomes increasingly difficult. Foran aerial image of low contrast, even those parts of the image thatcorrespond to the dark regions of the masks have significantintensities. Hence, exposure of the photoresist using a mask ofinsufficient contrast, causes even dark areas to be exposed to asignificant extent. On development, a blurred, poorly resolvedphotoresist is obtained.

A similar problem occurs in the optical projection of the mask image.The projected radiation has a tendency to scatter due to diffractionaround the edge of a light/dark area in the image. The farther inproximity printing, the image is from the resist, the greater the amountof radiation scatter under intended dark areas of the resist.

To improve contrast, a contrast enhancement layer is used in conjunctionwith an underlying positive or negative photoresist layer.

European patent application No. 110,165 filed on Oct. 29, 1983 in thename of Griffing and West discloses a contrast-enhancing layer used inconjunction with a photoresist layer. The contrast-enhancing layerconsists of a photobleachable material in a resinous binder. Thecontrast-enhancing layer is applied to the photoresist layer and forms asecond, in situ, mask upon exposure to light. Light travels through themask and acts on a photobleachable dye contained in thecontrast-enhancing layer. Areas of the contrast-enhancing layercorresponding to the mask pattern become transparent, and allow thephotoresist layer located below the contrast-enhancing layer to beselectively exposed to light according to the pattern of the mask.

Because a certain amount of light is required to render thephotobleachable dye transparent, the photoresist layer is exposed onlyafter the photobleachable dye has been made transparent. Dark areas ofthe mask allow less light transmission and, therefore, take longer tobleach the photobleachable dye of the contrast-enhancing layer.Correspondingly, less light is allowed through the bleached areas of thecontrast-enhancing layer to expose the photoresist layer in the darkareas. Hence, the contrast-enhancing layer effectively increasescontrast and the net result is a resolution improvement over using thepositive resist alone. After the photoresist and contrast-enhancinglayers have been exposed, the contrast-enhancing layer is stripped fromthe exposed photoresist with an organic solvent before development ofthe exposed photoresist.

Although the contrast-enhancing layer of Griffing and West is capable ofincreasing contrast and resolution, use of the layer in photolithographyrequires several extra steps which increase process time and reduceyield and efficiency.

It is an object of the present invention to provide a contrast-enhancinglayer which can be used in a standard aqueous photoresist developmentstep without requiring a separate contrast-enhancing layer strippingstep prior to photoresist development.

Another object is to provide a CEL that does not intermix with thephotoresist it coats.

Another object is to provide a CEL that shows a large decrease inabsorbance at 300-450 nm when exposed to those wavelengths.

Another object is to provide a more improved CEL layer capable offurther enhancing contrast and/or requiring shorter exposure times.

SUMMARY OF THE INVENTION

The present invention is directed to a contrast-enhancement compositionincluding at least one light-sensitive compound that produces an acidicphoto product on exposure to actinic radiation, at least one indicatordye that changes color in the presence of acid, and at least onepolymeric binder that is soluble in either water or weakly alkalineaqueous solutions.

Another aspect of the present invention relates to a photographicelement comprising a photoresist layer, a contrast-enhancement layerincorporating the above composition and a substrate.

Another aspect of the invention is directed to a method for producing animage in a photographic element by imagewise exposing this element toactinic radiation, the method comprising providing a contrast-enhancinglayer incorporating the above composition between a photoresist materialand a source of actinic radiation and exposing the photoresist materialby actinic radiation sufficient to photobleach the contrast-enhancementlayer and expose the underlying photoresist.

DETAILED DESCRIPTION OF THE INVENTION

The contrast enhancement layer (CEL) of the present invention includescompounds having the following characteristics:

a photosensitive compound that generates an acidic photoproduct onexposure to actinic radiation; an indicator dye that changes color inthe presence of acid; and at least one polymeric binder that is solublein either water or weakly alkaline aqueous solutions (pH about 7 orhigher).

The photosensitive compound used in the CEL should be sensitive toradiation at wavelengths of 300 to 450 nm. The desired concentration ofacidic photo-products produced on exposure to radiation is a function ofthe sensitivity of the indicator dye to pH change. Generally, it isdesirable that the photosensitive compound produce as large a decreasein pH of the layer as possible, so that the indicator changes colorfast.

Examples of the preferred photosensitive compounds that generate acidicphotoproducts include naphthoquinone-1,2(diazide-2)-5-sodium sulfonateand naphthoquinone-1,2-(diazide-2)-4-sodium sulfonate. Other suitablecompounds include the BF₄ ³¹ or PF₆ ⁻ salts of:

4-diazo-N-ethyl-N-hydroxyethylaniline;

4-diazo-N-methyl-N-hydroxyethylaniline;

4-diazo-2-chloro-N-N'-diethylaniline;

4-diazo-2-methyl-1-pyrrolidine benzene;

4-diazo-2,5-dibutoxy-N-benzoylaniline;

4-diazo-1-morpholinobenzene;

4-diazo-N-ethyl-2-toluidine;

4-diazodiethylaniline;

4-diazodimethylaniline;

4-diazodiphenylamine sulfate; and

4-diazo-2,5-diethoxyphenylmorpholine

These materials can be obtained from Fairmount Chemical Co., Inc.,Newark, N.J.

During exposure, the naphthoquinone diazide sulfonate (or other compoundlisted above) is converted to the corresponding indene carboxylic acid.For example: ##STR1##

The indicator dyes used should be highly absorbent at 300 to 450 nmuntil the CEL is exposed. The indicator dye should change color in thepresence of the indene carboxylic acid produced by the photoreaction. Inaddition, absorbance should disappear at 300 to 450 nm.

One advantageous feature of the CEL composition of this invention isthat the rate of photo-bleaching of the colored layer will increaseduring exposure. This increases the effectiveness of the contrastenhancement layer.

Two distinct, sequential, color-destructive reactions are believed tooccur when the present compositions are used in contrast-enhancinglayers. The first is believed to be the photoconversion of thenaphthoquinone diazide to the ketene intermediate (Wolff rearrangementreaction). For example: ##STR2## This reaction is believed to causedisappearance of absorbance in the region of 300 to 450 nm to the extentthat such absorbance is due to the naphthoquinone diazide.

The second is believed to be the reaction of the ketene with water toform an indene carboxylic acid, as follows: ##STR3## The acid will causethe indicator dye which is highly absorptive in the range of 300 to 450nm to become transparent to this wavelength.

A time lag exists between the decomposition of the naphthoquinonediazide and the change in color of the indicator dye. Thus, during theinitial light exposure, only the color from the naphthoquinone diazidewill disappear. But on continued exposure, (further) color bleachingwill result from the combined effects of the naphthoquinone diazidephoto-reaction and the indicator dye color change due to the presence ofacidic photo-products. Thus, the rate of color disappearance increasesas exposure continues.

The rate of color disappearance increases even faster (as exposurecontinues) for yet another reason: at the beginning of exposure, therate of photobleaching of the naphthoquinone diazide is slowed down bythe indicator dye which absorbs some of the incident light that wouldotherwise be available to bleach the naphthoquinone diazide. Laterduring exposure, the color of the indicator dye starts to disappear (inresponse to the formation of the acidic product). As a result, morelight is available to bleach the naphthoquinone diazide. This alsocauses the overall color disappearance to progress at a faster rate.

Under the conditions described above, the exposure latitude is increasedand contrast is further enhanced.

Examples of useful indicator dyes include 2,4-dinitrophenol,3-nitrosalicylaldehyde, metaniline yellow, methyl orange, ethyl orange,propyl red, ethyl red, methyl red, o-nitrophenol, m-nitrophenol,Na-p-nitrophenoxide, and pyridine-2-azo-dimethylaniline. Water solubleforms of the indicator dye are preferred. These dyes may be used aloneor in combination. The Na-p-nitrophenoxide is particularly preferredbecause it normally absorbs strongly at certain wavelengths (e.g. in theregion of 405-436 nm) but its absorbance at these wavelengths disappearsin acidic media.

The indicator dye and the photobleachable compound (e.g. thenaphthoquinone-diazide sulfonate) are held together in a polymericmatrix transparent to radiation at 300-450 nm. The polymer should besoluble in water or mixtures of water and water-miscible organicsolvents, and it should also be soluble in weak alkali. Moreparticularly, the polymer should be soluble in developing solutions usedto develop photoresist materials. Examples of preferred polymers includepoly(vinyl alcohol), poly(vinyl-pyrrolidone) andvinyl-pyrrolidone/vinyl-acetate copolymers.

The polymer, indicator dye, and photosensitive acid-producing compoundare dissolved in a solvent such as water, or a mixture of water and awater-miscible organic solvent. Examples of useful water-miscibleorganic solvents include methanol, ethanol, isopropanol, 2-methoxyethanol, dimethyl formamide and propylene glycol methyl ether. Thesolvent mixture should not attack the resist layer. The formation ofpinholes or interfacial layers is evidence of intermixing. Therefore,the coated resist should be checked to confirm the absence of pinholes.

The amount of the photosensitive acid-producing compound used in the CELcomposition is preferably in the range of about 5 to 90% by weight basedon the amount of the polymer, more preferably about 20 to 90% by weight.The amount of indicator dye used should be adjusted according to theacid produced by photobleaching of the acid-producing compound, so thatall of the indicator dye changes color. The indicator/photosensitivecompound ratio should be at least slightly less than 1:1. Preferably,the indicator will be present in the range of about 20-80% by weightbased on the amount of the photosensitive compound, more preferably inthe range of about 20-60%.

The solid components, photosensitive compound, indicator dye, andpolymer are dissolved in the solvent or solvent mixture in an amount ofabout 3 to 30 parts by weight, based on 100 parts by weight of thesolvent. The percent solids of the final CEL solution will depend on thecoating thickness desired and the spin speed used.

A small amount of one or more nonionic surfactants such as FC-431, ZonylFSN, or Surfynol 485 (obtained from 3M, St. Paul, MN; E. I. DuPont DeNemours & Co., Wilmington, Del.; and Air-Products Co., Allentown, Pa.,respectively) may be added to the CEL in the amount of about 0.01 to1.0% by weight to improve the surface properties of the solution.

The shelf-life of the CEL of the present invention is about one yearstored at room temperature.

The prepared CEL is spin-coated on top of a conventionalphotoresist-coated silicon wafer. The solvent is removed from the CEL byevaporation preferably in a convection oven at about 60° C.-90° C. forabout 5-15 mins. When exposed to light by projection through a mask at300 to 450 nm (monochromatic projection :G-Line --436 nm; H-Line--405nm; I-Line--367 nm), the CEL becomes selectively relatively transparentin a pattern corresponding to the light areas of the mask. The resistlayer is exposed for a period of time.long enough to render the CELtransparent in light mask areas only (time exposure would besource-dependent but is generally about twice that required for anuncoated resist); the dark areas of the mask remain dark on thecorresponding CEL area. Following exposure, the wafer having thephotoresist and CEL is developed by standard developing procedures asexplained in more detail below. During development, the CEL iscompletely stripped from the photoresist layer and a positive, resistimage of high resolution is obtained. A negative resist image can beobtained in the same way if an appropriate negative photoresist is used,as is well-known in the art.

The following examples illustrate the present invention but are notintended to limit its scope. Many modifications, additions, or deletionsmay be made within the scope of the invention and the invention includesall such modifications.

EXAMPLE I

A mixture of 2.4 parts by weight of poly(vinylpyrrolidone) (NP-K-30 GAFCorp.), 2.0 parts of naphthoquinone-1,2-(diazide-2)-5-sodium sulfonate(Fairmount Chem. Co.) 0.4 parts Na-p-nitro-phenoxide and 0.2 parts ZonylFSN (DuPont) was dissolved in a mixture of 61.2 parts deionized waterand 33.8 parts propyleneglycol methyl ether (from Dow Chemical Co.,Midland, Mich.), to obtain a CEL solution. The solution was filteredthrough a 0.2 micron membrane filter (from Millipore). The CEL solutionwas spin-coated on top of a conventional positive resist (such aspositive resist 119 from Fairmount) which was previously coated on anoxidized silicon wafer. The solvent was removed from the CEL followingcoating by baking the wafer for 10 minutes at 90° C. The wafer was nextimmersed into a bath of 3.5% aqueous solution of sodium metasilicate for30 seconds at 22° C., rinsed with water and dried. The CEL layer wascompletely removed during development in the alkaline solution and noevidence of intermixing (for example pinholes or interfacial layers) wasobserved on the positive resist layer.

EXAMPLE II

The CEL solution from Example I was coated onto a sheet of 0.3 gaugeMelinex 516, polyester film (from ICI, Wilmington, Del.). The solventwas removed by evaporation. A portion of the film was exposed for 5seconds using a 200 watt high-pressure mercury lamp. The absorbance ofthe exposed vs. the unexposed coating was measured at 405 and 436 nm.The ratio of: ##EQU1## was determined for each wavelength. At 436 nm theratio was 13.5 and at 405 nm the ratio was 29.5. These values indicatethat the CEL upon exposure to light had undergone an absorbance changewith absorbance disappearance at both 405 and 436 nm.

EXAMPLE III

The CEL solution of Example I was coated on top of a conventionalpositive resist (Positive Resist 119) coated wafer for 30 seconds at3000 rpm. A second positive resist-coated wafer was used as a controlleft uncoated.

The CEL coated wafer and the control were placed in an oven for 10minutes at 90° C. The two wafers were exposed for 2 and 4 seconds usinga 200 watt high pressure Hg lamp and a 21 step sensitivity guide. Thedeveloping conditions were 30 seconds in a 3.5% aqueous solution ofsodium metasilicate followed by a water rinse. The results aresummarized in Table I. The steps of a 21-step sensitivity guide aregraduated in increments of increasing density (from zero to +4).

The first open step achieved indicates that complete light exposure hadoccurred at this density. The first solid step indicates that thisdensity was opaque to light. On this basis, at approximately two timesthe exposure of the control the contrast-enhanced sample reproduces thesame exposure (that is an open step 3). At exposures in lower intensitythan that sufficient to expose a step 4, the light has been absorbed bythe CEL. This is evidence that contrast is enhanced.

                  TABLE I    ______________________________________    Exposure Time in Seconds                  2 Seconds 4 Seconds    CONTROL         Open 3, Solid 7                                Open 4-5, Solid 9    CONTRAST ENHANCED                    not done    Open 3-4, Solid 5    ______________________________________

EXAMPLE IV

An oxidized silicon wafer, 4 inches in diameter was placed on an SVG8032 CTD spinner (from Silicon Valley Group). Approximately 2 to 3 ml ofhexamethyldisilazane is placed in the center of the wafer. The wafer wasthen first spun at 500 rpm for 20 seconds followed by spinning at 5000rpm for 30 seconds. Hexamethyldisilazane is used to promote adhesionbetween the oxidized silicon wafer and the positive photoresist which isto be subsequently applied. Selectilux® P-ZLI-2692 which is a positiveresist (from E M Chemicals, Hawthorne, N.Y.) was then coated on top ofthe hexamethyldisilazane layer at a spin speed of 5000 rpm for 30seconds. After softbaking on a vacuum hot plate for 1 minute at 100° C.,the coating thickness was found to be 1.2 microns. The coated wafer wasimaged through a chromium mask in a projection scanning system availablefrom Perkin Elmer (Norwalk, Conn.) under the name Micralign® 300 HT.Aperture 1 was used and had equal lines and spaces from 5 micron linesand spaces to 1.5 micron lines and spaces. The scan setting was 520.After exposure the resist was developed for 60 seconds in a 2.1%solution of tetramethylammonium hydroxide at 22° C. Scanning electronmicroscope examination of the images showed that the 2.25 micron imageswere rounded on the top and that the side wall angles were approximately60° .

EXAMPLE V

The experiment of Example IV was repeated except that the contrastenhancement layer designated CEL-1 was coated on top of the photo-resistcoating at a spin speed of 3000 rpm. The coating was then softbaked at100° C. for 1 minute on a vacuum hot plate. After exposure on theMicralign® as previously described (except that the scan setting was440) the coating was developed according to Example IV.

During application of the contrast enhancement layer no intermixing hadtaken place between the photoresist and contrast enhancement layers. Thecontrast enhancement layer appeared to be a very uniform coating. TheCEL was also easily removed in the developer. Scanning electronmicroscope evaluation of the images shows that the contrast enhancementlayer improves resolution. The 2.0 micron images made with contrastenhancement are flat on the top surface and the side wall angles areapproximately 70°-75°. The resolution with the contrast enhancementlayer within the 2.0 to 2.25 micron range is improved by at least 0.25microns.

What is claimed is:
 1. A contrast-enhancing, photobleachable compositionfor use on a photoresist layer comprising:a water-soluble,light-sensitive compound which, upon exposure to actinic radiation,produces acidic photoproducts and its absorbance of radiation in therange of about 300 to about 450 nanometers substantially disappears,said compound being selected from the group consisting ofnapthoquinone-1,2-(diazide-2)-5-sodium sulfonate andnaphthoquinone-1,2-(diazide-2-4-sodium sulfonate; a water-solubleindicator dye which before exposure to acidic conditions is highlyabsorbent to radiation within the range of about 350 to about 450 nm,but upon exposure to acidic conditions, undergoes a substantialdisappearance of its absorbance of radiation in said range, said dyebeing selected from the group consisting of 2,4-dinitrophenol,3-nitrosalicylaldehyde, metaniline yellow, methyl orange, ethyl orange,propyl red, ethyl red, methyl red, o-nitrophenol, n-nitrophenol,sodium-p-nitrophenoxide and pyridine-2-azodimethylaniline; and apolymeric binder which is substantially transparent to radiation in therange of about 300 to about 450 nanometers and is soluble in a memberselected from the group consisting of water and weakly alkaline aqueoussolutions;Wherein said compound is provided in an amount ranging betweenabout 5 and about 90% by weight based on said polymeric binder and saidindicator dye is provided in an amount not exceeding the maximum amountthat can be rendered transparent to said radiation range by said acidicphotoproducts of said compound.
 2. The composition of claim 1, whereinsaid binder is soluble in the developing solutions used for thedevelopment fo photoresist materials.
 3. The composition of claim 2wherein said polymeric binder is slected from the group consisting ofpoly(vinyl alcohol), poly(vinyl pyrrolidone) and vinyl pyrrolidone-vinylacetate copolymers.
 4. The composition of claim 1 wherein said indicatoris sodium-p-nitophenoxide.
 5. The composition of claim 3 wherein saidpolymeric binder is selected from the group consisting of poly(vinylalcohol), poly(vinyl pyrrolidone) and vinyl pyrrolidone/vinyl acetatecopolymers.
 6. The composition of claim 5 further comprising a solventselected from the group consisting of water, watermiscible organicsolvents and mixtures thereof.